That is very possible. It would be interesting to see them do the laptops first, and not unexpected, as that's where they are the most beind. I still think the initial reports of a mini based on the P-M are more likely, tho. Get something out quickly that is fast, cheap, and still stylish, stable, easy to use, etc, and they may sway the doubters with it.
Releasing an iBook built on that same platform, and a Powerbook with upgraded specs shortly after (like 3 months or so) is my next guess. After people see the mini as fast, compatible, good value, etc, they will be willing to plop down a little extra money for a new portable.
Plus, bringing the mini out first will help, as it will (hopefully) sell well and get more Intel machines out there to convince developers this will work. Also, I would bet a lot of mini users use MOSTLY software that is already going to be available for 'Universal Binary' release. iLife, iWork, Office, Photoshop (Elements mostly), Safari and Mail (of course), etc. The change in CPU will be completely transparent to them.